Conventionally, there have been known motor drive devices that have a function of detecting insulation resistance of motor winding by applying a voltage charged on a smoothing capacitor in a direct current (DC) link unit, between the motor winding (coil) and the earth (e.g., Japanese Patent No. 5065192 (JP 5065192B), which will be referred to hereinbelow as ‘Patent Document 1’ and Japanese Laid-open Patent publication No. 2012-233826 (JP 2012-233826A), which will be referred to hereinbelow as ‘Patent Document 2’). Patent Document 1 discloses a motor drive device having an insulation resistance detecting function. In this conventional motor drive device, after shutting off an alternate current (AC) power supply by a switch, a smoothing capacitor of a DC power supply (DC link unit) connected to the inverter unit is connected at its one end to the earth while a plurality of semiconductor switching elements connected to the other end of the smoothing capacitor are turned on, one by one in a predetermined order. By this operation, a closed circuit formed of the smoothing capacitor, the ground, the motor coil and the on-state semiconductor switching element is created to thereby detect the current flowing through this closed circuit with a current detection circuit and determine the insulation resistance of a motor.
Further, Patent Document 1 discloses a method of detecting insulation resistance of a motor to be detected, in a motor drive device equipped with a plurality of inverter units for driving a plurality of motors. First, an arbitrary motor to be detected is selected from a plurality of motors, and only the semiconductor switching elements of the inverter unit to which the motor selected to be detected is connected are turned on. Next, the semiconductor switches of all of the inverter units to which a motor other than the target for measurement is connected are kept off. As a result, a closed circuit that passes through the insulation resistance of the motor to be detected among the plural motors is formed so that the insulation resistance of the detection-target motor is detected.
Patent Document 2 discloses a method of detecting insulation resistance of a motor to be measured that is able to realize detection of insulation resistance similar to that in Patent Document 1, also in a motor drive device that uses bootstrap circuits in the drive circuitry for semiconductor switching elements in an upper arm of the inverter unit. First, as to at least a pair of upper and lower arm semiconductor switching elements in the inverter unit to which the motor to be measured is connected, one switching element is turned on while the other is turned off. Then the first one is turned off while the second is turned on. This on-and-off operation is repeated by use of a PWM (Pulse Width Modulation) signal of the same duty ratio. With this operation, a closed circuit passing through the insulation resistance of the motor coil of the motor to be measured is formed so as to detect the insulation resistance of the motor to be measured based on the current flowing through this closed circuit and the voltage across the smoothing capacitor.
In Patent Document 1, the semiconductor switching elements of the inverter unit to which the motor to be measured are kept on-state at the time of measurement. In contrast to this, Patent Document 2 is different in that repeating switching operation is performed, or the semiconductor switching elements in the upper and lower arms of the inverter unit to which the motor to be measured is connected are alternately turned on by use of the PWM signal.
In Patent Document 2, the reason why the switching operation in which the upper and lower arm semiconductor switching elements are alternately turned on and off is that since the drive circuit for the semiconductor switching element of the upper arm is formed of a bootstrap circuit, the upper arm semiconductor switching element becomes unable to be turned on unless the bootstrap circuit for the upper arm semiconductor switching element is charged by turning on the lower arm semiconductor switching element.
The embodiment of Patent Document 2 describes that insulation resistance is measured by applying a positive power supply from the smoothing capacitor to the motor coil of the motor to be measured during the period in which the upper arm semiconductor switching element is turned on, then charging operation of the bootstrap circuit of the upper arm semiconductor switching element is performed during the period in which the lower arm semiconductor switching element is turned on.
In order to measure insulation resistance, only the semiconductor switching elements of the inverter unit to which the motor selected as a detection target is connected, are turned on while all the other semiconductor switching elements of the inverter units to which a motor other than the target for measurement is connected are kept off-state. In this way, a closed circuit that passes through the insulation resistance of the detection-target motor among the plural motors is formed so as to measure the insulation resistance of the detection-target motor. This is the common feature both in Patent Document 2 and Patent Document 1.
In the prior art described in Patent Document 1 and Patent Document 2, for the motor drive device having a plurality of intervener units for driving a plurality of motors, the semiconductor switching elements originally equipped in the inverter units are used as the selector switches for selecting a detection-target motor from plural motors. Therefore, it is not necessary to separately provide as many switches as the number of motors in order to select a detection target. Further, since insulation resistance can be measured on the plural motors by use of a single detecting circuit, the prior art methods are excellent in being able to realize measurement with simple and low-cost configurations.
However, the prior art methods described in Patent Document 1 and Patent Document 2 entail the problem that when, in the motor drive device including a plurality of inverter units that drive a plurality of motors, a specific detection-target motor is selected from plural motors to measure insulation resistance, if even one of the motors other than the detection target is low in insulation resistance, the measurement accuracy lowers in a high-temperature condition under which leakage current through semiconductor switching elements increases.
FIG. 1 illustrates a configuration of a motor drive device 1000 including two inverter units for driving two motors, according to the prior art disclosed in Patent Document 1. In the example of FIG. 1, description will be made on a case where a first motor 1061 is selected from the two motors as a target for measurement so that the insulation resistance of the first motor 1061 is measured.
The procedures of measuring the insulation resistance of a motor using the conventional motor drive device are as follows. To begin with, in FIG. 1, while all of semiconductor switching elements (IGBTs) 10511 to 10561, 10512 to 10562 in all inverter units 1051 and 1052 have been turned off, a first switch 1001 is turned off so as to cut off an AC power supply 1002. Next, a second switch 1009 is turned on so that a minus-side terminal 1043 of a DC link unit 1004 is connected to the earth. FIG. 2 represents an equivalent circuit of the connection of insulation resistance between the IGBTs and the motor in the above condition.
Next, as a result of the first motor 1061 having been selected as a target for measurement, the IGBT 10511 of the U-phase upper arm of the first inverter unit 1051 to which the first motor 1061 is connected is turned on so as to create a closed circuit (indicated by the broken line in FIG. 1) passing through the insulation resistance between motor coils 10611-10631 of the first motor 1061 to be measured and the earth to thereby measure the current flowing through the closed circuit by means of a current detector 1007. At the same time, the DC link voltage is measured by a voltage detector 1008 to determine the insulation resistance between the motor and the earth from the measured voltage and current.
FIG. 3 depicts the equivalent circuit at the time of this measurement of insulation resistance. Since one of the upper arm IGBTs (10511, 10531, 10551) in the first inverter unit 1051 is changed to off-state from the state illustrated in FIG. 2, FIG. 3 is the equivalent circuit of the circuit illustrated in FIG. 2 in which the equivalent insulation resistance RU-IGBT1 of the upper arm IGBTs (10511, 10531, 10551) in the first inverter unit 1051 is short-circuited.
In FIGS. 2 and 3, RU-IGBT1 and RU-IGBT2 represent the equivalent insulation resistances of upper arm IGBTs (10511, 10531, 10551) in the first inverter unit 1051 and upper arm IGBTs (10512, 10532, 10552) in the second inverter unit 1052 in their off-state, respectively; RD-IGBT1 and RD-IGBT2 represent the equivalent insulation resistances of lower arm IGBTs (10521, 10541, 10561) in the first inverter unit 1051 and lower arm IGBTs (10522, 10542, 10562) in the second inverter unit 1052 in their off-state, respectively; Rm1 and Rm2 represent the insulation resistances between the motor coils (10611 to 10631) of the first motor 1061 and between the motor coils (10612 to 10632) of the second motor 1062 and the earth, respectively; and Rc represents the series of a voltage dividing resistance 1072 of the current detector 1007 and a current detection resistance 1071 as a single resistance.
In the case of a three-phase inverter for driving a three-phase motor illustrated in FIG. 1, one inverter unit includes three semiconductor switching elements (IGBTs) in each of the upper and lower arms. The three IGBTs of the upper arm as well as the lower arm in one inverter are connected in parallel with their collector terminals joined together with the DC link unit and their emitter terminals joined via the motor coils inside the motor. For this reason, the three IGBTs in the upper arm or in the lower arm in the inverter are represented as a single equivalent insulation resistance in the equivalent circuits in FIGS. 2 and 3.
In the prior art, in the motor drive device including a plurality of inverter units for driving a plurality of motors, when insulation resistance is detected by selecting a specific motor to be measured, the leakage currents flowing through the insulation resistance of the motors other than that the target for measurement and the off-state semiconductor switching elements connected to those motors are superposed over the measurement current flowing through the current detector. As a result, in particular, when the motor drive device have a large number of motors connected to hereto and if the insulation resistances of the motors other than the target for measurement are lowered, there occurs the problem that the measurement accuracy in the insulation resistance measurement of the specific motor to be measured markedly lowers in a high-temperature condition under which leakage currents through semiconductor switching elements increase.
In the above description, ‘the leakage current flowing through the off-state semiconductor switching element’ in the example of an IGBT, corresponds to the leakage current flowing from the collector to the emitter when the IGBT is in off-state.
This leakage current in off-state is defined as an electric characteristic represented by a symbol ICES in IGBT, and is called ‘collector-emitter leakage current’. The collector-emitter leakage current (ICES) is defined as the leakage current flowing from the collector to the emitter when the gate and the emitter are short-circuited, i.e., when the rated voltage is applied between the collector and the emitter with the IGBT totally shut off.
The IGBT collector-emitter leakage current (ICES) is greatly dependent on temperature, specifically, the leakage current ICES increases exponentially with a rise in temperature.
The characteristic of increase in leakage current at off-state with a rise in temperature is not limited to IGBTs, but it has been known that a similar characteristic is found also in other semiconductor switching elements such as MOS-FETs, and the like. For example, the similar electric characteristic in a case of MOS-FET is defined as the leakage current between drain and source at off-state, which is represented by a symbol IDSS.
In general, the reason why increase of leakage current ICES in IGBTs for use in inverters for motor drive is regarded as a problem is mainly in view of loss increase. However, when IGBTs in the inverter unit are used as the selector switches for the motor to be measured on insulation resistance as in the prior art disclosed in Patent Document 1 and Patent Document 2, even a leakage current ICES as low as a level of some tens μA, which will not cause any problem in view of loss, will cause degradation of measurement accuracy in the prior art measurement of insulation resistance of motors.
Specifically, as apparent from FIG. 3, the problem of the prior art resides in that superposed over the actual current (the dashed line with arrows in FIG. 3) to be measured, flowing through the insulation resistance Rm1 between the first motor to be measured and the earth, part of the leakage current flowing through the off-state semiconductor switching element RU-IGBT2 connected to the second motor other than the target for measurement directly flows into the current detector 1007 by way of the insulation resistance Rm2 of the second motor other than the target for measurement (the dashed-and-dotted line with arrows in FIG. 3).
If the semiconductor switching element is an ideal selector switch, it can be considered that as long as the semiconductor switching element is set in off-state, the motors other than the target for measurement and the DC link unit are separated by the off-state semiconductor switching elements so that no current will flow. However, since the actual semiconductor switching element even in its off-state will permit leakage current to flow at a level that affects the measurement accuracy of the insulation resistance measurement when a voltage is applied in a high-temperature condition, a great caution has to be given.
The leakage current that generates measurement error flows through the semiconductor switching element RU-IGBT2 of the inverter unit connected to the second motor other than the target for measurement and the insulation resistance Rm2 of the second motor other than the target for measurement, as illustrated in FIG. 3. Accordingly, even with the prior art, if the summed insulation resistance of the insulation resistance Rm2 of the second motor other than the target for measurement and the equivalent insulation resistance of the semiconductor switching element RU-IGBT2 connected to the second motor is sufficiently high compared to the insulation resistance Rm1 of the first motor to be measured, the measurement accuracy of the insulation resistance Rm1 of the first motor to be measured will not become so low as to pose a practical problem.
However, when considering the use purpose of exactly measuring the insulation resistance of every motor by switching the target for measurement for all the plural motors and identifying a motor if there is any that is lowered in insulation resistance, it is considered in practical operation that there are many cases where there are some motors that are lowered in insulation resistance, among the plural motors.
If there is a motor that is lowered in insulation resistance, there necessarily occurs the case where the motor lowered in insulation resistance is included in the motors other than the target for measurement in the course of measuring the insulation resistance of every motor by switching the target for measurement. Further, since the semiconductor switching element connected to the motor that has been lowered in insulation resistance could present a low equivalent insulation resistance due to high temperature, high-accuracy measurement is demanded for such cases.
It is assumed that as a motor other than the target for measurement a motor with degraded insulation resistance as low as, for example 1 [MΩ] is connected. In this case, if, as a tentative value, the total resistance of the equivalent insulation resistance of the semiconductor switching element in its off-state plus 1 [MΩ] is sufficiently greater than the insulation resistance of the motor to be measured, it is not considered that any trouble that produces serious influence on measurement accuracy will occur. However, when the total resistance is approximately equal to or lower than the insulation resistance of the motor to be measured, it is practically impossible to achieve high-accuracy measurement of insulation resistance.
FIG. 4 is a graph that represents the behavior of the collector-emitter leakage current (ICES), specifically, a leakage current flowing through an IGBT with a withstanding voltage of 1200 [V] that is typically used in the industrial inverter units, depending on the temperature.
FIG. 4 is a graph that has been obtained by measuring leakage current in circuitry where three IGBTs in the upper arm are connected in parallel with their collectors joined together and emitters joined together, on the assumption of application to a three-phase inverter unit. Similarly to this, the graph obtained by the measurement with the three lower arm IGBTs connected in parallel, exactly coincides with the graph for the upper arm. Therefore, the graph in FIG. 4 is represented by a single line.
Table 1 is a table showing equivalent insulation resistances of the IGBT between collector and emitter at different temperatures, determined by dividing the applied voltage, 1200 [V] between collector and emitter, by the leakage current flowing from collector to emitter, read from the graph in FIG. 4.
TABLE 1IGBT's EquivalentIGBT's JunctionIGBT's LeakageInsulationTemperatureCurrent ICESResistance 25° C. 0.3 μA 4 GΩ 80° C.  40 μA30 MΩ100° C. 200 μA 6 MΩ
Referring to FIG. 4 and Table 1, description will be made on how the leakage current of the IGBT at each temperature gives influence on the measurement of insulation resistance of the prior art.
At normal temperature (25[° C.]) the leakage current in the IGBT's off-state is as low as 0.3 [μA], which corresponds to an equivalent insulation resistance of about 4 [GΩ]. This value is sufficiently high compared to the insulation resistance of the motor to be measured (100 [MΩ] to 1 [MΩ]). Accordingly, even if a motor with an insulation resistance of 1 [MΩ] or below is connected as one of the motors other than the target for measurement, it is considered that there is little influence on measurement accuracy of the insulation resistance of the motor at normal temperature.
However, as the temperature of IGBT becomes higher, the leakage current becomes greater exponentially. At a junction temperature Tj of 80[° C.], the IGBT leakage current ICES is 40 [μA], this corresponding to an IGBT equivalent insulation resistance of 30 [MΩ]. In this case, if the insulation resistance of any of the motors other than the target for measurement is lowered to about 1 [MΩ], this level of insulation resistance will influence the accuracy of measuring the insulation resistance of the motor by the prior art method.
Further, when the junction temperature Tj rises up to 100[° C.], the leakage current ICES in the IGBT's off-state increases to about 200 [μA], which corresponds to an equivalent insulation resistance of about 6 [MΩ]. In this case, even if the insulation resistance of 1 [MΩ] of the motor other than the target for measurement is added, the sum is as low as, or becomes lower than, the insulation resistance value of the motor to be measured. As a result, it is practically difficult to measure insulation resistance with high accuracy.
As described heretofore, in the case where IGBTs having the characteristic illustrated in FIG. 4 are used, if the insulation resistance of any of the motors other than the target for measurement is lowered, accurate detection of insulation resistance can be only done in a limited temperature range, or at around normal temperature or below in the prior art. For example, in a high temperature state directly after the operation of the motors by the inverter units, if even one of the motors other than the target for measurement has a lowered insulation resistance, it is understood that there occurs a problem that the accuracy of detecting the insulation resistance of the motor to be measured is deteriorated especially when high insulation resistance is measured.
Even with the prior art disclosed in Patent Document 1 and Patent Document 2, in the motor drive device including a plurality of inverter units for driving a plurality of motors, only when all the motors connected to the motor drive device are made to be the target for measurement so that the insulation resistance of all the motors is collectively measured, at least one of the plural semiconductor switching elements of the inverter unit connected to each motor is in on-state at the time of measurement so that there is no inverter unit in which all the semiconductor switching elements are in off-state. As a result, the problem of lowered measurement accuracy will not occur theoretically due to influence of the leakage current flowing through a semiconductor switching element in its off-state as described above, hence it is possible to achieve high-accuracy measurement.
The measurement result obtained by this method of measuring all the motors collectively, gives the combined resistance of all the insulation resistances connected in parallel. Accordingly, if the measured value of insulation resistance obtained by this method is at a sufficiently high level free from problems, it is possible to determine that the insulation resistances of all the motors are free from insulation deterioration.
However, when the measured value of insulation resistance obtained by this method is at such a low level as to pose a problem, it is possible to know that at least one of the motors has been lowered in insulation resistance, but it is impossible to obtain any information to identify which motor has been degraded in insulation resistance.
In order to identify which motor among the plural motors has been degraded in insulation resistance, it is necessary to measure insulation resistance by selecting a specific motor, one by one, from all the motors. However, as described above the prior art measurement in this condition entails the problem that measurement accuracy is degraded especially when the temperature is high.
The motor drive devices having an insulation resistance detecting function for motors are built in machine tools and others and mainly utilized for maintenance and preservation activities of the tools on production sites in factories. In the preservation activities on the production site in the factory, when insulation degradation has taken place in any one of plural motors, the task of finding the fact of occurrence is important. However, in order to trouble-shoot it is also important to identify which motor has been degraded in insulation, at the same time.
In the measurement for identifying the motor that is degraded in insulation resistance from plural motors, the prior art problem, in particular, the degradation of measurement accuracy at the time of high temperatures, has been demanded to improve from an operational viewpoint on the production site.
The present invention has been devised in view of the problems described above, it is therefore an object of the present invention to provide a motor drive device including a plurality of inverter units for driving a plurality of motors, which can be constructed with a simple structure, which, when the insulation resistance of a specific motor among the plural motors including a motor degraded in insulation resistance is measured, enables exact measurement of the insulation resistance of the motor and correct detection of insulation deterioration even at high temperatures by using semiconductor switching elements originally provided for the inverter units as the selector switches for selecting a specific motor from the plural motors, without being affected from leakage currents flowing through semiconductor switching elements connected to the motors other than the target for measurement.